1. Field of the Invention
The present invention relates to a magnet roller to deliver a developer to a latent image support element used in an image forming apparatus such as a copier, a facsimile machine, or a printer. It also relates to a developer holder element including the magnet roller, a develop unit including the developer holder element, a process cartridge including the develop unit, and an image forming apparatus including the process cartridge.
2. Description of the Prior Art
Generally, an electrophotographic image forming apparatus generates visual images by forming an electrostatic latent image on a latent image support element as a photoconductor drum or belt based on image information and developing it with a develop unit. Magnetic brush developing is widely used in such an electrophotographic develop process. For example, with use of two-component developer, the developer is magnetically attracted onto an outer circumference of a developer holder element as a develop roller to form a magnetic brush. By electric field in a develop area between the latent image support element and the developer holder element, toner is attracted from the magnetic brush onto a latent image on the latent image support element.
A develop roller used in the magnetic brush developing comprises a cylindrical develop sleeve made of non-magnetic materials and a magnet roller contained in the develop sleeve to form developer chains on the surface of the develop sleeve by a magnetic force. Chains of magnetic carrier contained in the developer are formed on the develop sleeve along magnetic field lines (magnetic force) of the magnet roller and toner is attracted to the chains of magnetic carrier.
Electric color copiers and color printers are now widespread and they generally need four develop units for four colors (yellow, magenta, cyan, black). To downsize an image forming apparatus, needless to say that the develop unit and the develop roller contained in the develop unit have to be downsized.
Downsizing of the develop roller is realized by decreasing diameter of the magnet roller. However, a smaller magnet roller generally made of ferrite resin includes a less magnet volume so that it fails to generate necessary magnetic force.
Aiming to solve such a problem, Japanese Patent Application Publication No. 2010-8471 (Reference 1) discloses a magnet roller including a columnar magnetic field generator of ferrite resin with a groove on the outer face in the axial direction in which a high magnetic power earth magnet block is fixed. This magnet roller is of a small diameter but can generate high magnetic force.
However, when the magnetic field generator of this magnet roller is molded by magnetic field injection molding, a periphery of the groove and a portion facing the groove may be hardened and shrunk. This may cause a problem that due to a difference in the shrinkage of the two portions, the magnetic field generator warps. Such a warped magnet roller cannot generate magnetic field evenly in the axial direction in a develop process and deliver the developer evenly, causing a degradation in image quality.
Japanese Patent No. 3826622 (Reference 2) discloses a warpage correction device for a magnet roller configured to locally cool down a predetermined portion of a magnet roller removed from a mold in incompletely hardened state while rotating the magnet roller. This device locally cools down a portion of the outer circumference of the magnet roller opposite by about 180 degrees to a warped concave portion to create a hardened layer. Then, it cools down the entire magnet roller to mainly shrink the semi-hardened concave portion and negate the warpage of the magnet roller.
Another problem with the magnet roller having the groove in Reference 1 is that depending on a diameter of the magnet roller, the groove in which the earth magnet block is placed need be formed to be deeper than the positions of support portions of the magnet roller. This leads to weakening the support portions since the groove is shrunk when molded and further causing a breakage or an inclination of the support portions immediately after the molding, in assembly or during use of the magnet roller.
The warpage correction device disclosed in Reference 2 has a problem that stress may occur inside the magnet roller due to the local cool-down and cause a distortion therein over time. Similarly to the warped magnet roller, such a distorted magnet roller cannot generate magnetic field evenly in the axial direction in a develop process and deliver the developer evenly, causing a degradation in image quality. Also, another process to attach the magnet roller to this warpage correction device has to be added in the manufacturing process, increasing manufacture costs.
In order to prevent the breakage or inclination of the support elements of the magnet roller, Japanese Patent Application Publication No. 2009-217208 (Reference 3) discloses a magnet roller having a groove whose depth about the support elements is shallower than the diameter of the support elements. However, there still remains the warpage problem unsolved. Especially, for the purpose of generating a high magnetic force at a main magnetic pole, the magnet roller in a small diameter has to be provided with a groove deeper than the diameter of the support elements into which a high power magnet block is placed. This is likely to bring about warpage of the magnet roller.